The present invention is related to the field of brushless direct current motor control systems, and more particularly to such systems which include protection circuitry that prevents the destruction of control system components by the occurrence of larger than expected signals which are coupled to the control system components.
Many types of brushless direct current motor control systems are known, and they may be used in automotive applications for controlling fuel pump and fan motors. One known control system is illustrated in U.S. Pat. No. 4,403,177 to Weber et al which is assigned to the same assignee as the present invention. In this prior motor control system, in response to a switch closure low frequency oscillator and sequencing circuits are activated which provide cyclic and sequential phase drive signals to three phase windings of a direct current motor. The sequential actuation of the phase windings results in the rotation of a permanent magnet rotor which in turn results in inducing voltage signals in the two of the three phase windings which are not currently being excited. The phase windings are typically configured in a Y configuration with the induced signals varying about a reference level maintained at the neutral (common) point of the Y configuration. The magnitude of the induced voltages is monitored by a motor control circuit such that when these induced signals exceed a predetermined magnitude related to the voltage potential at the neutral terminal, sequencing of the phase winding drive signals occurs in accordance with the induced signals. Thus, while initially a low frequency oscillator provides for initial sequencing of the phase winding drive signals, subsequently the induced voltage signals in the phase windings are utilized to determine the sequencing of phase winding excitation and thereby determine the rotation of the motor. In this manner the brushless direct current motor is properly driven and controlled without utilization of Hall effect or various other motor position rotation sensors. Similar direct current motor control systems are described in detail in U.S. Pat. Nos. 4,262,236 and 4,262,237.
For proper operation of motor control systems such as those discussed above, it is necessary for the motor control circuit to perform an accurate comparison of the phase induced voltages provided in response to the rotation of the permanent magnet rotor with respect to the nominal voltage level potential about which these phase induced voltages vary. This nominal voltage level exists at the neutral point of the Y configuration of the phase windings and this neutral point is typically directly connected to a direct current battery reference potential. When the nominal voltage of the battery may vary, such as is the case in automotive applications, it is necessary that the motor control circuit receive, as an input signal for comparison with the phase induced voltages, an input reference signal which substantially tracks the reference signal level at the neutral phase winding terminal. This is necessary because unless the motor control circuit receives the proper input reference signal the small signals that are induced in the phase windings during starting will not be properly detected, resulting in continuous operation in the starting mode.
In systems such as that described above, it is possible to directly connect the neutral terminal of the Y configuration as a sense input to the motor control circuit. However, in that case no protection is provided for the motor control circuit in the event of high voltage transient signals occuring at the neutral terminal, and also no protection is provided in the event that a substantially greater than nominal DC voltage is provided at the battery terminal which is connected to the phase winding neutral terminal.
In order to guard against both of these eventualities, a prior protection circuit has been proposed comprising a series resistor coupled between the neutral terminal and the input terminal of the motor control circuit and including a zener diode coupled between ground potential and the input terminal of the motor control circuit. This prior protection circuit has proved to be unsatisfactory because typically the input terminal of the motor control circuit also provides all of the DC operating current to the motor control circuit such that a relatively low impedance to ground is provided at this input terminal. This results in the series resistor providing a variable potential drop between the common terminal and the input terminal of the motor control circuit wherein this potential drop varies in accordance with the amount of current drain provided by the motor control circuit and the magnitude of the battery voltage coupled to the neutral terminal. Thus the input signal to the motor control circuit did not just vary as a function of the magnitude of the signal level at the neutral terminal, but also was determined by the input impedance of the motor control circuit which typically would vary between different motor control circuits. This was unsatisfactory since it would require the individual adjustment of each motor control circuit to provide the proper comparison threshold for the control circuit comparison of the phase induced signals with respect to the voltage at the neutral terminal of the Y configuration. If the control circuit DC operative power terminal was separate from the sense input terminal, then an additional overvoltage circuit might be needed for the operative power terminal thus increasing the system cost, also, if the control circuit was an integrated circuit, an additional input lead for the integrated circuit would be required. In order to overcome all these disadvantages the present invention has been developed.